The original claim was that the founders were "antifascists."

Personally, I wouldn't call them fascist or antifascist because I disagree with the definition that extends fascism back to include all monarchies and such. I prefer to use a narrower definition of it, because the conditions of Germany, Italy, and other fascist countries were very different from the feudal system that had existed further back in the past, and it's more useful and accurate to have a word that describes those conditions specifically. Otherwise, I think we're diluting the term and making it much more nebulous.

Trying to fit the founders into one of these boxes of "fascist" or "antifascist" is projecting modern politics into a historical situation where it doesn't really apply, rather than simply seeing what was. That broad way of thinking is something I consider wrong and dangerous, whatever the categories are. For example, during the Cold War, the US saw the world in terms of "communist" or "not communist" and everyone, everywhere in the world had to fit into one of those categories. Anti-colonial struggles, such as in Iran, were labelled as communist even when they weren't. Reality is far more complex than such broad oversimplifications allow for.

The British system, for example, was a constitutional monarchy (still is) where power was shared between the monarchy and parliament. But even before parliament, feudal systems were more complex than just "whatever the king says goes," there were layers upon layers of contracts between each level of noble that said what they could and couldn't do, and a king that violated those contracts would likely face a rebellion from the nobility. Meanwhile, the system in Nazi Germany was designed to encourage different branches of government competing against each other, making it somewhat less cohesive and centralized than is often imagined.

I'm going to genuinely try here. They were never trying to actually argue the founding fathers were antifa.

They threw out two possibilities.

1. A colonial Empire can be considered antifa as long as it fight fascists, even if it does foced labor.

2. Only anarchists can be antifa, "welcome to the right side of socialism. "

You ignored the last bit, signaling their actual stance, and decided to trash the first possibility as stupid, excluding any state from the definition of antifascist. The followup is the pointing out that China and Russia therefore are not antifascist. You tried to handwave it but then got pushed into defending the American prison system because it's better than chattel slavery. It's not an incorrect stance; the systems in China, Russia, US all suck but they're better than they used to be. I think that's essentially where wyvern was trying to end up.

Summed up: if you don't believe China and Russia were/are antifascist simply because they oppose the US, you both agree.

That's the best you've got?